Listen Now Download as mp3 from the show Analysing Antimatter QuestionAyush, India asked: If light is massless, how does it bend due to gravity? AnswerWe posed this question to Andrew Pontzen from the University of Cambridge... Andrew  Regardless of the mass of the object, the acceleration caused by gravitational pull is the same for any object. Now, Newton came along and gave a mathematical explanation of this and the maths essentially is that mass appears on both sides of the equation which governs this behaviour. So it actually cancels out. But if the mass is actually zero, then it’s no longer really mathematically valid to do that cancellation. Nonetheless, it’s certainly true experimentally and mathematically that as you go to smaller and smaller masses, these things are still deflected in the same way by gravity. But since this sort of mathematical paradox of trying to divide by zero, that isn’t conclusive. To get the full mathematical answer actually requires coupling a description of what we call electromagnetic waves, that's the kind of physics underlying the wave light travels, to Einstein’s theory of gravity which is general relativity. Only then do we get rid of this paradox of dividing by zero and end up with a conclusive answer that shows that just as objects of any mass are affected by gravity. So light which has no mass is also affected by gravity. Diana  So, what is it that relativity tells us about gravity that can help us solve the problem? Andrew  So in the end, Einstein’s description of gravity which is general relativity tells us that the effect of gravity is caused by distortions in space and time itself. Now if you do something as fundamental as distorting space and time, and reshaping it, anything that lives inside space and time will be affected. That includes waves, and so, waves can be bent and can follow different paths if you change the geometric properties of the space they live in. Diana  Gravity can effectively bend space and time. Meaning that anything in its field is also distorted and that includes light. Multimedia
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CommentsExcellent question, and I will await the answer. As I understand it, the photon only has a relativistic mass when travelling at or near the speed of light. This is a massive simplification  and even this is hard to visualize  but here goes.... In essence; gravity comes from mass and energy which distort the whole of space and time  light still travels the shortest distance within this spacetime, but as spacetime is curved it now follows a geodesic not a deadstraight line. It's because a photon posesses acceleration. Acceleration and curvature and gravity are all ensembles of the same thing in relativity, so the acceleration of a photon couples to the gravitational curvature of spacetime. QuantumClue, Tue, 18th Jan 2011 I recently read.......all of the ancient debates concerning religion, science and philosophy come down to to just a single mathematical question  does zero have actual existence or is it just a mathematical construct that never appears in reality?Science and religion will at last be reconciled when scientists stop waging their crazy war against zero. One day soon they will see the light  light being the quintessence of the zero domain, as Einstein proved. light is both dimensional and dimensionless it is massless and experiences no distance and no time when it travels so is everywhere all at once That would be like saying that understanding a human concept like zero is to understand the all and all of physics. This is not true. QuantumClue, Thu, 20th Jan 2011 between 0 and infinity lies the totality of physics:........ I read this...... My gran used to say i was her ray of light. Ayush, light doesn't 'bend'. But it depends on how you see it. I look at space as a 'topology', which in a way is a daft thing to do :), as I then assign a shape to a 'nothing'. And space is a 'nothing', really it is. If we assumed gravity to be something like magnetism we could speak of it 'attracting' light, but that's not the truth, there is nothing EM about gravity as far as I know. Haven't read other answers but I'll have a go anyway. QuantumClue stated that a photon possesses acceleration. Perhaps I am misunderstanding the point, but I thought that a photon travels at the speed of light so how does it accelerate? Also, acceleration is inversely proportional to mass, so calculating a massless particle's acceleration would require division by zero. Ron Maxwell, Mon, 24th Jan 2011
Think of the photons as being accelerated by gravity. That was easier for me./ QuantumClue, Tue, 25th Jan 2011 you have to think of space as a plane and gravity exerted by any heavy object as a bowling ball being dropped on a plane. it drops but doesn't break. now light when passing next to that heavy mass will be deviated from its path because space itself is warped Lethalwolf, Tue, 25th Jan 2011 A photon do not 'accelerate'. Instead it will 'blue shift' from the observer inside a 'gravity well'. and when leaving it 'red shift' according to an observer outside as well as inside that gravity well. Light have only one 'speed', not several. But where 'speed' fails energy comes in. Both the blue shift and the red shift will express itself as more, or less, 'work done' when measured by the observers described. Perhaps that spacebending anecdote given by Einstein has the answer... Space near mass is denser, and gravity is actually refraction. Faye_Kane, Thu, 24th Feb 2011 If light is massless, how does it bend due to gravity? Heikki http://www.bbc.co.uk/news/scienceenvironment12620560 huh? CZARCAR, Fri, 4th Mar 2011 Gravity is a geometry. The geometry express itself everywhere, inside us and outside. Quantum physics hopes to find evidence for it becoming 'particles' of some kind, like the Higgs boson/field or gravitons. But, as far as I understand it, Einstein didn't see gravity as 'particles'. Geometry is shapes, everything radiation shows us has a 'shape' of some kind, except 'space'. I think you might be havering Yoron. Sorry, what's the argument Geezer? Maybe you visualize light as being visible streaks through 'space'? And then it is 'convoluted' and so if we only could straight it out we would get more 'distance' from it? And so we could argue that 'space' exist? If you won't agree with me that distance is a discussable concept then show me where and how it can't be disputed please. To me it seems as if both particles and space builds on the same concept, 'distance'. Then we have some things that seems to make it without. Light. yor_on, Sat, 2nd Apr 2011 When light bends to gravity then that's because it plays by the 'rules'. The 'rules' we have state that everything obeys 'gravity'. Space and matter seems very alike in that they both contain this property, 'distance'. Then we have some things that don't. Light is what comes to my mind then. Particles are also defined by their need to take up a '3Droom' wherever they are, sort of reserving a 'space'. That's also why we expect them to be there tomorrow. Light on the other side just 'is'. That you can find it everywhere tells you nothing about a speed or a 'distance'. Light only exist in a interaction, that we use our ideas of a '3D room geometry in times arrow' to measure a 'speed' is a direct result from where we live. No guarantee of this being anything else than a defect in something else, getting 'locally' (SpaceTime) some very weird properties enabling linearity inside nonlinearity inside linearity inside ... light does not bend (but it still can be considered as bending) the space and time around light is bended making it seem like light is bending hence light is bending and not bending at the same time even if light were not affected by gravity it would still bend due to gravitational time dilation. granpa, Sun, 17th Apr 2011 My understanding is that about 10 years ago the weight of photons was measured. My other question is; is the fourth dimension dark matter? David, Sat, 12th Jul 2014 
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